1. Field of the Invention
The present invention relates to photoelectric conversion devices, image reading apparatuses, image forming apparatuses and methods of photoelectric conversion.
2. Description of the Related Art
A pixel signal output from a photoelectric conversion unit of an image sensor is converted into a digital signal by an A/D converter and is output to a downstream stage. There is a need to prevent an image signal (an electrical signal) from being saturated on the bottom side (a black side) of the A/D converter even when noise is generated in a pixel and a processing circuit.
For example, in Japanese Unexamined Patent Application Publication No. 2005-101985, disclosed is a solid-state imaging apparatus that includes, in a clamp system, a black-level detection circuit that detects a minimum black level of a serial signal and a clamp-level setting circuit that sets a clamp level based on the minimum black level. The set clamp level is, after analog conversion, fed back to a clamp circuit in a column processing circuit or an analog front-end circuit, and the black level of a signal that passes through is changed there.
However, there has been a problem in that, when an offset is given to a plurality of A/D converters from one end side of arrayed pixels, the wiring for giving an offset voltage tends to be long and the distribution of the offset voltage arises due to the influence of the impedance of the wiring. Because a line sensor generally is of a pixel size larger than that of an area sensor and is formed in a rectangular shape longer in one direction, there has been a problem in that the tendency of the distribution of the offset voltage to arise is further increased.